1. Technical Field
The invention relates generally to a method and apparatus for encapsulating objects carried by a lead frame strip in an encapsulating material. More specifically, the invention relates to a method and apparatus for encapsulating electronic components such as semiconductor chips in plastic.
2. Background Art
Although not limited thereto, the present invention has particular suitability to the encapsulation of electronic components. For example, in the manufacture of semiconductor devices, it is common practice to provide lead frames in strip form, the strip having chip support pads spaced along the length thereof and leads extending lengthwise along the strip from adjacent to each pad. A semiconductor chip is then supported on each pad and electrical connections are made from the various leads to each chip. After this has been done, the strip is then placed in a mold having a cavity for each chip and a suitable plastic encapsulating material is then forced into the mold to encapsulate each chip, a portion of the leads therefrom and the electrical connections of the leads to the chip.
Typical molds now in use will accommodate eight lead frame strips, each having ten lead frames thereon, so that 80 semiconductor devices may be encapsulated on one shot. Other molds can handle twelve strips of ten lead frames each for simultaneous encapsulation of 120 devices.
Regardless of the particular number of devices to be encapsulated at a time, the typical present day mold comprises an upper and a lower mold member. With the upper mold member raised, the various lead frame strips are positioned on the lower mold member such that the portions to be encapsulated are in registration with the mold cavities formed in the lower mold member, there being, of course, one cavity for each of the strip portions. The upper mold member, which also has a complementary cavity formed thereinto for each strip portion, is then lowered onto the lower mold member.
The mold is further provided with a main injection opening through one of the mold members which communicates with relatively large feed runners that extend along the length of the strips, with relatively long gate runners which branch off of the feed runners and extend to each cavity. A suitable plastic, in liquid form, is then injected through the main opening, to fill the feed runners, gates and mold cavities.
After the plastic has hardened, the mold members are separated and the strips, with the encapsulating portions thereon, are removed from the mold. To prevent damage to the devices as the mold members are separated, ejection pins are provided on at least one of the mold members to engage the encapsulating devices and force them out of the mold cavities thereof during mold separation.
The molding process is then repeated with a new batch of lead frame strips.
Although the present molds provide an effective encapsulation, they have a number of definite disadvantages. A substantial amount of downtime exists. Each time a mold is used, the cavities, runners and gates must be checked and cleaned out, since a clogged runner or gate or a partially filled cavity will prevent the next molding operation from being carried out successfully. Time is also required to position each batch of lead frame strips in the mold prior to molding and to remove them from the cavities after molding. Additionally, it is customary to shut down for about an hour each day for a thorough cleaning of the mold.
The mold members are expensive, and require a considerable amount of precision machining to form the many cavities and the plastic distribution runners. Likewise, because of the many cavities and runners, they are difficult to clean and are relatively easy to damage during the usual mechanical cleaning thereof.
Typically, the plastic which is used hardens upon the polymerization thereof. As a consequence, the injected plastic which fills the distribution system and hardens therein is pure wastage--it cannot be reused. Because of the extensive distribution systems of present molds, the ratio of plastic in the distribution system to the plastic actually used for encapsulation is high and the wastage of the relatively expensive plastic material is substantial.
Present molds are also expensive because of the ejection pin systems. Typically, two ejection pins are provided for each device to eject it from the cavity in which it is formed. Thus, for a 120 cavity mold, 240 ejection pins are required. Further, the pins will cause surface deformation of the plastic where the pins engage the devices, detracting from the appearance of the finished product as well as providing less room on the surface thereof for application of legending or other indicia thereto.
In U.S. Pat. No. 4,332,537, and improved molding apparatus and method is disclosed which does not suffer from many of the drawbacks of the typical prior art molds. The mold and molding method taught in that patent utilize a cavity plate means which is removably positioned between first and second mold plates with the cavity plate flush against the first mold plate. The cavity plate means holds objects spaced from the first mold plate and has openings therethrough in registration with the portions of the objects which are to be encapsulated. Fluid plastic is forced through feed runners formed in the first mold plate and outwardly from the mold plate and into the openings of the cavity plate means to encapsulate the portions of the objects held thereby. As a result, the objects can be premounted on the cavity plate means prior to positioning of the cavity plate means between the mold plates. As a further result, the lower mold plate is completely flat for easy cleaning. Yet another result is that the feed runners in the first mold plate and the openings in the cavity plate means can be located to enable the maximum number of objects to be encapsulated for a given mold area. Still another result is that ejection pins may not be needed at all and if such are used they will be used only to remove hardened plastic from the feed runners of the first mold plate when the mold plates are moved apart for removal of the cavity plate means.
While an apparatus as disclosed in U.S. Pat. No. 4,332,537 solves the aforementioned problems of the typical molds of the prior art, room for further improvement still exists. For example, downtime is not reduced as much as would be desirable since the feed runners tend to stick to the cavity plate means. Also, cleaning off of the feed runners from the cavity plate means tends to produce undesirable wear on the cavity plate means. Further, it would be desirable to use smaller molds for producing equal numbers of encapsulated components because significant energy savings would result since much less energy is needed to heat a smaller mold to operating temperature than is needed to heat a larger mold to the same temperature. Still further, additional savings in plastic are desirable, not only because plastic is expensive, but also because making the plastic requires the expenditure of sizeable amounts of energy. Of even greater importance, because of the necessity for cleaning off the feed runners, the encapsulation mold of U.S. Pat. No. 4,332,537 is not ideal for adaptation for automation.